1. Field of the Invention
The present invention relates to a method of forming fixed images used for plain paper copying machines, laser printers, plain paper facsimiles, etc. More particularly, it relates to a method of forming fixed images in which low temperature fixing is carried out using a thermally dissociating encapsulated toner.
2. Discussion of Related Art
Conventionally, when images are formed with copying machines, laser beam printers, etc., the Carlson Method has been generally used (U.S. Pat. Nos. 2,221,776, 2,297,691 and 2,357,809, "Electrophotography," p22-p41, R. M. Shaffert, 1965, The Focal Press).
FIG. 6 is a schematic view of an apparatus used in a conventional method of forming fixed images. In the conventional method of forming fixed images, after the electrostatic latent image formed on a photoconductor by optical means is developed in a developing process, it is transferred to a recording medium such as recording paper in a transfer process and then fixed into the final image generally with heat and pressure in a fixing process. As the photoconductor is repeatedly used, a cleaning device is provided for cleaning the residual toner after the transfer process during its rotation.
In the conventional method of forming fixed images, however, the processes from the formation of the electrostatic latent image up to its fixing onto the recording medium are time consuming, which makes the apparatus used therein not only complicated but also large. In addition, since the transfer efficiency of the toner is poor in the transfer process, it poses such problems as extra labor needed for the disposal of the toner collected by cleaning the residual toner, and the pollution due to the scattering of the toner in and out of the apparatus.
Specifically, in the conventional method of forming fixed images, through the processes from the formation the electrostatic latent image up to the fixing of the developed image onto the recording medium, the temperature of the heating element of the fixing device has to remain at a very high level (usually around 200.degree. C.) and further a relatively high nip pressure is required (usually between 2.0 and 6.0 kg/cm). On the other hand, since both the photoconductor and the developing device have to be maintained at around room temperature, a considerable distance has to be maintained between the fixing device and the developing device, which necessitates making the machine larger. In addition, it is necessary to forcibly remove the generated heat from the system, but the noise produced by the forced radiation device is quite noticeable.
Therefore, a method of simultaneously conducting transferring and fixing has been proposed (U.S. Pat. No. 4,448,872). In this method, since the transferring and the fixing are simultaneously carried out by pressing the toner image developed on the dielectric drum to the recording medium, simplification of the apparatus can be achieved. However, since only pressure is applied at the fixing, the fixing ability is poor, and only little improvement is achieved in the transfer efficiency.
The fixing of the toner should be generally conducted at a high temperature due to the high melting temperature of the toner, thereby requiring an apparatus with a high thermal efficiency. The fixing process usually works independently, and is carried out at a high temperature of around 200.degree. C. Accordingly, expensive heat-resistant materials, such as heat-resistant resins, heat-resistant rubbers, etc. have to be provided in the periphery of the fixing device.
When the fixing is carried out at a high temperature, it is subject to problems such as curling and jamming of the paper, etc. In addition, it is pointed out that fixing failure may take place due to the heat absorbed by the paper, depending upon its thickness. Further, if the fixing requires a high temperature, it takes more time to reach the set temperature so that quick printing becomes impossible. In such a case, therefore, this method is unsuitable for devices such as a facsimile which requires quick printing.
Further, in view of solving these problems, there has been proposed a method of forming fixed images, wherein the transfer and fixing process is simultaneously carried out by adhering the toner onto a transfer film, which rotates while keeping it partly in close contact with a photoconductor to form a toner image, and putting the recording paper and the transfer film between a pressure roller and a heat roller provided away from the photoconductor (Japanese Patent Laid-Open No. 197884/1990).
According to this method, however, when the transfer film is wound around the pressure roller, one of the pair of rollers for transfer and fixing, and a heat roller is arranged on the outside of the transfer film, the recording medium, which is heated from the reverse side, such as paper, etc. having an insulating effect has a poor thermal efficiency, and thus sufficient heat required for fixing cannot be supplied to the toner. Therefore, problems arise in that fixing becomes insufficient. On the other hand, when the transfer film is wound around the heat roller and the pressure roller is arranged on the outside of the transfer film, the heating material is arranged in the inside of the film belt, causing problems in radiation from the internal portion of the film belt. When the heating material is arranged inside the film belt, the radiation conditions are likely to be insufficient, thereby causing deterioration in sensitivity and decrease in durability of the photoconductor due to heat. In addition, since the fixing is carried out through the film, problems may arise in the delaying of the transmission of heat, thereby presumably demanding a higher fixing temperature for the heat roller.
Also, there has been known a method of using a photoconductive film as a photoconductor, wherein after a developing process, the transfer and fixing are carried out on the recording medium by heating the photoconductive film adhered with the toner from the reverse side (Japanese Patent Laid-Open No. 36581/1991). However, it has been pointed out that since the usual toner is used in this method, the deterioration of the photoconductive film due to heat may take place, making it unsatisfactory to meet the requirements.
From these standpoints, the development of a novel method of forming fixed images as well as a matching toner thereto is in demand.